The present invention is directed to novel high performance filters having characteristics suitable for use in various filtration applications, such as microfiltration. More particularly, this invention is directed to a novel high performance filter media comprising a blend of conventional carbon, ceramic, glass or silica fibers and inorganic fiber whiskers that is low in cost, durable, resistant to chemicals and high temperatures, not subject to particulation, high in mechanical strength and separation efficiency, and biocompatible.
The prior art provides many types of materials which remove, filter, or capture gases and particulate materials. These filters of the art, while fairly effective in the applications for which they were designed, do not offer the efficiency, performance, and durability demanded by new, high performance applications.
The demand for higher quality materials, reduced manufacturing costs, and environmentally clean processes is forcing industry to move away from traditional methods of separation and purification, such as distillation and pasteurization, towards the use of filtration. Filter systems are now capable of offering low energy, more efficient, and environmentally friendly operations. Unfortunately, the widespread use of high performance filtration is restricted by the lack of suitable filter media materials. Such media must offer low cost; durability; chemical resistance, particularly to acids and alkalis; resistance to high temperatures, for both operation and sterilization purposes; no particulation (i. e., release of filter media particles into the filtrate stream); mechanical strength to cope with pressure swings; separation efficiency, particularly for particles in the 0.1 to 100 microns range; and biocompatibility for certain applications such as the filtration of blood.
Table 1 below lists the currently available filter media materials together with their advantages and disadvantages.
As presented in Table 1, no one filtration material offers the required balance of properties needed for new, high performance applications.
Ceramic filter media have made some inroads, however their acceptance is hampered by the following: high cost because expensive and complex manufacturing processes are required; susceptibility to attack by alkalis; limited durability because of their inherent brittleness; and difficulties in controlling pore size distribution and permeability, which are critical aspects of high performance filter media. High temperature composite materials, in which a ceramic or carbon matrix is reinforced with a continuous fiber, are used in a variety of applications. They are most commonly used in aircraft brakes. In this application, the braking material is made from a carbon matrix reinforced with carbon fibers (carbon/carbon or C/C). Such materials have a high mechanical strength and are capable of operating at extreme temperatures, up to 3000xc2x0 C. in a non oxidizing atmosphere. Composites in which both the reinforcing fiber and the matrix are both ceramic are used in specialty applications. In particular, they are used in aircraft engine parts where strength at high temperatures and low weight are needed.
Such high temperature composite materials do offer some potential for use as filter media. For example, carbon/carbon composites, due to the excellent balance of properties, have found use as a filter support. U.S. Pat. No. 4,944,996 discloses the use of a carbon/carbon support intended to receive a mineral membrane for separation procedures. U.S. Pat. No. 4,500,328 discloses the use of carbon/carbon composites to filter radioactive waste, and the use of activated carbon fiber to increase surface area. U.S. Pat. No. 5,183,546 discloses an electrochemical filter consisting of an electrically conductive fibrous material that contains microscopic particles of carbon or active charcoal.
Ceramic matrix composites have been used as hot gas filters. U.S. Pat. No. 4,968,467 discloses the use of refractory ceramic fibers matted together with a high temperature binder, such as colloidal alumina or silica, to form a tube like xe2x80x9ccandle filter.xe2x80x9d U.S. Pat. No. 5,196,120 discloses the use of a ceramic fiber-ceramic composite filter composed of ceramic fibers, preferably texturized, a carbonaceous layer thereover, and a silicon carbide coating over the carbonaceous layer, which coats substantially all of the fibers. A strong, light weight filter is achieved.
Despite the advances made in the art, of which the above are examples, ceramic and carbon based composite materials have not previously been suited to high performance filtration. This is especially true for microfiltration because of the difficulties in achieving the required porosity, surface area and permeability required for efficient separation. In general, pore size distribution and the ability of the filter to retain or capture particulate matter is a function of the fiber diameter (Filters and Filtration Handbook, Third Edition, 1992). In the art outlined above, fiber diameters range from 7 microns for conventional carbon fibers to 100 microns and above for some ceramic fibers. The diameters of such fibers are too large. These fibers do not provide the small pores required for efficient small particulate retention. U.S. Pat. No. 5,138,546 discloses the addition of small carbon or charcoal particles which improves surface area and particle capture ability. However, this type of filter is not suitable for most high performance applications, particularly in the foodstuffs and chemical industries. These structures exhibit poor bonding of the particles to the substrate. In addition, there is a tendency for such constructions to particulate, in other words, release undesired particles into the filtrate stream. Also, the addition of such particles can only be performed on a random basis. There is little control with respect to uniformity and positioning.
In light of the disadvantages of the prior art, there is therefore presently a need to develop high performance filter media capable of operating in the microfiltration regime which offer: low cost; durability; chemical resistance, particularly to acids and alkalis; resistance to high temperatures, for both operation and sterilization purposes; no particulation (i. e., release of filter media particles into the filtrate stream); mechanical strength to cope with pressure swings; separation efficiency, particularly for particles in the 0.1 to 100 micron range; and biocompatibility for certain applications, such as the filtration of blood.
It is therefore an object of the present invention to provide a filter media having a high degree of mechanical integrity and stiffness, which is capable of resisting pressure changes with pulsed flows.
It is another object of the present invention to provide a filter media having controlled pore size.
It is another object of the present invention to provide a filter media having a high level of permeability.
It is another object of the present invention to provide a filter media having a high flow rate through the filter media.
It is another object of the present invention to provide a filter media having a low pressure drop across the filter media.
It is another object of the present invention to provide a filter media, which in the microfiltration range, provides a pore size distribution sufficiently small to trap most particles in the range of about 0.05 to about 10 microns.
It is another object of the present invention to provide a filter media that offers substantial versatility in construction so that a variety of constructions can be produced in order to cope with different filter applications.
It is another object of the present invention to provide a filter media that is easily cleaned, and that has a similar retention efficiency after the cleaning process.
The present invention therefore provides a filter media, including a structure comprising a blend of inorganic fibers and inorganic fiber whiskers. The filter media of the present invention is low in cost, durable, resistant to chemicals and high temperatures, not subject to particulation, high in mechanical strength and separation efficiency, biocompatible, has a high permeability, a high flow rate and a low pressure drop across the filter media.
The present invention further provides a method for preparing a filter media structure comprising mixing together inorganic fibers, inorganic fiber whiskers, an organic binder and water to form a slurry; removing excess water from said slurry by filtration; drying said slurry at a temperature sufficient to fuse the binder to form said filter media structure; and optionally heating said filter media structure at a temperature sufficient to burn off or carbonize the binder. In one embodiment, a thin layer of pyrolytic carbon is deposited on the surface of said inorganic fibers and said inorganic fiber whiskers to anchor said fibers and fiber whiskers together, further strengthening the high temperature filter media structure.
The present invention provides a further method for preparing the filter media structure comprising inorganic fibers and inorganic fiber whiskers comprising mixing together inorganic fiber whiskers and a resin, impregnating said inorganic fibers with said inorganic fiber whisker/resin mixture to form a random assemblage of inorganic fibers and fiber whiskers; carbonizing said resin/inorganic fiber whisker impregnated inorganic fibers; and optionally depositing a thin layer of pyrolytic carbon on the surface and inside said inorganic fiber whisker/resin mixture to strengthen the filter media structure.